Data communication in a computer network involves the exchange of data between two or more entities interconnected by communication links and sub-networks. These entities are typically software programs executing on hardware computer platforms, such as end stations and intermediate stations. Examples of an intermediate station may be a router or switch which interconnects the communication links and subnetworks to enable transmission of data between the end stations. A local area network (LAN) is an example of a subnetwork that provides relatively short distance communication among the interconnected stations; in contrast, a wide area network (WAN) enables long distance communication over links provided by public or private telecommunications facilities.
Communication software executing on the end stations correlate and manage data communication with other end stations. The stations typically communicate by exchanging discrete packets or frames of data according to predefined protocols. In this context, a protocol consists of a set of rules defining how the stations interact with each other. In addition, network routing software executing on the routers allow expansion of communication to other end stations. Collectively, these hardware and software components comprise a communications network and their interconnections are defined by an underlying architecture.
Modern communications network architectures are typically organized as a series of hardware and software levels or "layers" within each station. These layers interact to format data for transfer between, e.g., a source station and a destination station communicating over the network. Specifically, predetermined services are performed on the data as it passes through each layer and the layers communicate with each other by means of the predefined protocols. The lower layers of these architectures are generally standardized and are typically implemented in hardware and firmware, whereas the higher layers are generally implemented in the form of software running on the stations attached to the network. Examples of such communications architectures include the Systems Network Architecture (SNA) developed by Inteerational Business Machines Corporation and the Internet communications architecture.
The Internet architecture is represented by four layers which are termed, in ascending interfacing order, the network interface, internetwork, transport and application layers. These layers are arranged to form a protocol stack in each communicating station of the network. FIG. 1 illustrates a schematic block diagram of prior art Internet protocol stacks 125 and 175 used to transmit data between a source station 110 and a destination station 150, respectively, of a network 100. As can be seen, the stacks 125 and 175 are physically connected through a communications channel 180 at the network interface layers 120 and 160. For ease of description, the protocol stack 125 will be described.
In general, the lower layers of the communications stack provide internetworking services and the upper layers, which are the users of these services, collectively provide common network application services. The application layer 112 provides services suitable for the different types of applications using the network, while the lower network interface layer 120 of the Internet architecture accepts industry standards defining a flexible network architecture oriented to the implementation of LANs.
Specifically, the network interface layer 120 comprises physical and data link sublayers. The physical layer 126 is concerned with the actual transmission of signals across the communication channel and defines the types of cabling, plugs and connectors used in connection with the channel. The data link layer, on the other hand, is responsible for transmission of data from one station to another and may be further divided into two sublayers: Logical Link Control (LLC 122) and Media Access Control (MAC 124).
The MAC sublayer 124 is primarily concerned with controlling access to the transmission medium in an orderly manner and, to that end, defines procedures by which the stations must abide in order to share the medium. In order for multiple stations to share the same medium and still uniquely identify each other, the MAC sublayer defines a hardware or data link address called a MAC address. This MAC address is unique for each station interfacing to a LAN. The LLC sublayer 122 manages communications between devices over a single link of the network and provides for environments that need connectionless or connection-oriented services at the data link layer.
Connection-oriented services at the data link layer generally involve three distinct phases: connection establishment, data transfer and connection termination. During connection establishment, a single path is established between the source and destination stations. This connection, e.g., an IEEE 802.2 LLC Type 2 or "Data Link Control" (DLC) connection as referred hereinafter, is based on the use of service access points (SAPs); a SAP is generally the address of a port or access point to a higher-level layer of a station. Once the connection has been established, data is transferred sequentially over the path and, when the DLC connection is no longer needed, the path is terminated. The details of such connection establishment and termination are well-known and, thus, will not be described herein.
The transport layer 114 and the internetwork layer 116 are substantially involved in providing predefined sets of services to aid in connecting the source station to the destination station when establishing application-to-application communication sessions. The primary network layer protocol of the Internet architecture is the Internet protocol (IP) contained within the internetwork layer 116. IP is primarily a connectionless network protocol that provides internetwork routing, fragmentation and reassembly of datagrams and that relies on transport protocols for end-to-end reliability. An example of such a transport protocol is the Transmission Control Protocol (TCP) contained within the transport layer 114. Notably, TCP provides connection-oriented services to the upper layer protocols of the Internet architecture. The term TCP/IP is commonly used to refer to the Internet architecture.
Data transmission over the network 100 therefore consists of generating data in, e.g., sending process 104 executing on the source station 110, passing that data to the application layer 112 and down through the layers of the protocol stack 125, where the data are sequentially formatted as a frame for delivery onto the channel 180 as bits. Those frame bits are then transmitted over an established connection of channel 180 to the protocol stack 175 of the destination station 150 where they are passed up that stack to a receiving process 174. Data flow is schematically illustrated by solid arrows.
Although actual data transmission occurs vertically through the stacks, each layer is programmed as though such transmission were horizontal. That is, each layer in the source station 110 is programmed to transmit data to its corresponding layer in the destination station 150, as schematically shown by dotted arrows. To achieve this effect, each layer of the protocol stack 125 in the source station 110 typically adds information (in the form of a header field) to the data frame generated by the sending process as the frame descends the stack. At the destination station 150, the various encapsulated headers are stripped off one-by-one as the frame propagates up the layers of the stack 175 until it arrives at the receiving process.
SNA is a mainframe-oriented network architecture that also uses a layered approach. The services included within this architecture are generally similar to those defined in the Internet communications architecture. In a SNA network, though, applications executing on end stations typically access the network through logical units (LU) of the stations; accordingly, in a typical SNA network, a communication session connects two LUs in a LU-LU session. Activation and deactivation of such a session is addressed by Advanced Peer to Peer Networking (APPN) functions.
The APPN functions generally include session establishment and session routing within an APPN network. FIG. 2 is a schematic block diagram of a prior art APPN network 200 comprising two end stations 202, 212, which are typically configured as end nodes (EN), coupled to token ring (TR) subnetworks 204, 214, respectively. During session establishment, an EN (such as EN 202) requests an optimum route for a session between two LUs; this route is calculated and conveyed to EN 202 by an intermediate station functioning as a network node server (e.g., station 206) via a LOCATE message exchange through the network 200. Thereafter, a "set-up" or BIND message is forwarded over the route to initiate the session. The BIND includes information pertaining to the partner LU requested for the session.
Intermediate session routing occurs when the intermediate stations 206, 216, configured as APPN network nodes (NN), are present in a session between the two end nodes. As can be seen, the APPN network nodes are further interconnected by a WAN 210 that extends the APPN architecture throughout the network. The APPN network nodes forward packets of an LU-LU session over the calculated route between the two APPN end nodes. An APPN network node is a full-functioning APPN router node having all APPN base service capabilities, including session services functions. An APPN end node, on the other hand, is capable of performing only a subset of the functions provided by an APPN network node. APPN network and end nodes are well-known and are, for example, described in detail in Systems Network Architecture Advanced Peer to Peer Networking Architecture Reference IBM Doc SC30-3422 and APPN Networks by Jesper Nilausen, printed by John Wiley and Sons, 1994, at pgs 11-83.
FIG. 3 is a schematic block diagram of the software architecture of a prior art APPN node 300. As noted, application 302 executing on an APPN end node, such as EN 202 of network 200, communicates with another end node, such as EN 212, through a LU-LU session; LU 304 within each end node functions as both a logical port for the application to the network and as an end point of the communication session. The session generally passes through a path control module 312 and a data link control (DLC) module 316 of the node, the latter of which connects to various network transmission media.
When functioning as an APPN router node, such as NN 206, an intermediate session routing (ISR) module 305 maintains a portion of the session in each "direction" with respect to an adjacent network node, such as NN 216 of network 200. In response to receiving the BIND message during session establishment, path control 312 and ISR 305 are invoked to allocate resources for the session. In particular, each NN 206, 216 allocates a local form session identifier (LFSID) for each direction of the session; the LFSID is thereafter appended to the packets in a SNA transmission header (TH) to identify the session context. Collectively, each of these individually-established "local" sessions form the logical communication session between the LUs 304 of the end nodes 202, 212.
When initiating a session, the application 302 specifies a mode name that is carried within the BIND message and distributed to all APPN network nodes; the LU 304 in each node uses the mode name to indicate the set of required characteristics for the session being established. Specifically, the mode name is used by control point (CP) module 308 of each APPN node 300 to find a corresponding class of service (COS) as defined in a COS table 310. The CP coordinates performance of all APPN functions within the node, including management of the COS table 310. The COS definition in table 310 includes a priority level specified by transmission priority (TP) information 320 for the packets transferred over the session; as a result, each APPN network node is apprised of the priority associated with the packets of a LU-LU session. The SNA architecture specifies four (4) TP levels: network priority, high priority, medium priority and low priority. Path control 312 maintains a plurality of queues 314, one for each TP level, for transmitting packets onto the transmission media via DLC 316.
Data link switching (DLSw) is a forwarding mechanism for the SNA architecture over an IP backbone network, such as the Internet. A heterogeneous DLSw network is formed when two DLSw switches interconnect the end nodes of the APPN network by way of the IP network; the DLSw switches preferably communicate using a switch-to-switch protocol (SSP) that provides packet "bridging" operations at the LLC (i.e., DLC) protocol layer. FIG. 4 is a schematic block diagram of a prior art DLSw network 400 comprising DLSw switches 406, 416 interconnecting ENs 402, 412 via IP network 410. The DLSw forwarding mechanism is also well-known and described in detail in Request for Comment (RFC) 1795 by Wells & Bartky, 1995 at pgs 1-91.
According to the DLSw technique, a lower-layer DLC connection is established between each EN and DLSw switch; however, these connections terminate at the switches 406, 416. In order to provide a complete end-to-end connection between the end nodes, the DLC connections are "disposed" over a reliable, higher-layer transport mechanism, such as TCP sessions. DLSw switches can establish multiple, parallel TCP sessions using well-known port numbers. All packets associated with a particular DLC connection typically follow a single, designated TCP session. Accordingly, SNA data frames originating at a sending EN 402 are transmitted over a particular DLC connection along TR 404 to DLSw switch 406, where they are encapsulated within a designated TCP session as packets and transported over IP network 410. The packets are received by DLSw switch 416, decapsulated to their original frames and transmitted over a corresponding DLC connection of TR 414 to EN 412 in the order received by switch 406 from EN 402.
Typically, all packets transmitted by DLSw switch 406 over a DLC connection/TCP session flow at the same priority level from a single output queue 405 of the switch and arrive at an output queue 415 of DLSw switch 416 in the same order in which they are transmitted. When the switches are configured as bridges to forward packets over a TCP session through the IP network, prioritization is straightforward. However, it may be desired to integrate the functions of an APPN network node within switch 406 by overlaying an APPN layer onto a DLSw layer of the switch; the resulting hybrid node may prioritize the packets at the APPN layer in an order governed by the TP information levels.
A problem that arises when deploying a hybrid node in such a heterogeneous network is that the TP priority information is lost when passing the packets between the APPN and DLSw layers, primarily because the TP information is not encapsulated within the packets. That is, the APPN layer has knowledge of the TP levels associated with the packets of a LU-LU session as a result of the BIND message exchange during session establishment; yet that information is not encapsulated within the associated packets and, thus, is not conveyed beyond the APPN layer. An example of a tagging mechanism suitable for use with the present invention that conveys TP levels from the APPN layer to the DLSw layer is disclosed in copending and commonly-assigned U.S. patent application, titled Technique for Maintaining Prioritization of Data Transferred Among Heterogeneous Nodes of a Computer Network, filed herewith and incorporated by reference as though fully set forth herein.
As described in the commonly-assigned application, the APPN protocol layer of the hybrid node assigns a TP level to each packet and passes that priority information to the DLSw layer of the node via an application programming interface extension. The TP level is converted to information that is "tagged" to each packet and the DLSw layer allocates each tagged packet to a TCP session based on the assigned TP level. The tagged information is then encapsulated within an IP header to enable intermediate routers to maintain the order and priority of the packet as it is transmitted outbound over the IP network to a receiving DLSw switch.
However, the tagged information within the IP header is not discernible to the receiving DLSw switch and, thus, the switch has no knowledge of the TP level associated with the outbound packet. If that packet requests a response, the DLSw switch cannot select, on the basis of priority, the proper TCP session over which to transmit a corresponding inbound packet; accordingly, the switch arbitrarily chooses a session. If the chosen TCP session has a lower designated priority than the session carrying the outbound packet, network throughput may be negatively impacted.
One solution to this problem is to deploy another hybrid node in place of the receiving DLSw switch. This approach is undesirable primarily because a goal of heterogeneous network design is to minimize the number of hybrid nodes in the network. A reason for minimizing the number of hybrid nodes is that such nodes require additional processing and memory resources, thereby resulting in expensive deployments. The present invention is directed to solving the problem of distributing packet prioritization information, assigned by a hybrid node of a heterogeneous network, to switching nodes of the network.